;]()({ PROFESSOR KELLAND ON FRESNEL'S FORMULA FOR THE 



ture and results of the molecular hypothesis, I return to the examination of the 

 memoir, which appears in some points to argue against it. 



Mr Green states, that two waves will result from giving a motion to a fluid, 

 such as that commonly supposed to he the medium the vibrations of which con- 

 stitute light, the one transversal and the other normal. On a careful examina- 

 tion of his memoir, I cannot discover this normal vibration ; the nearest approach 

 to it appears to result from the circumstance, that two waves, the incident and 

 the reflected, may be transmitted at the same time, and therefore cross each other. 

 If, then, in this case, the angle of incidence be an angle of 45°, one vibration may 

 be at right angles to the other ; but this circumstance does not in the slightest 

 degree militate against any conclusions which have been arrived at by the mole- 

 cular hypothesis. The coexistence of vibrations travelling in different directions, 

 is distinctly recognised in that theory. It may be well to state clearly, that the 

 point, and I think a most important one, which has been proved from the mole- 

 cular hypothesis, is this ; that 07ie wave cannot consist partly of normal, partly 

 of transversal vibrations. Of course, the definition of the wave restricts it to a 

 state of motion transmitted in one direction with one velocity. 



There can be little doubt, however, that the normal vibration to which Mr 

 Green refers, is supposed to be contained in that function which he introduces 

 in the body of his memoir, as the result of the change of motion from an incident 

 and reflected to a refracted one. This vibration is, however, merely a vibrator}'' 

 motion, not transmitted in the same direction as the incident ; and in the sequel 

 of the present memoir, it will appear that it is really and bond fide a transverse 

 vibration. Thus a statement, which at the first sight appears to argue powerful- 

 ly against the molecular theory, does, when attentively examined, afford strong 

 presumptive evidence in its favom-. 



I have deemed it right to be explicit on this subject, as the admission of Mr 

 Green's statement, if it left hypotheses such as Laplace's as to the constitution 

 of media uninjured, would absolutely crush the more probable hypothesis of the 

 Newtonian law of gravitation applied to the ultimate atoms. 



There is another point in Mr Green's paper which, although not so im- 

 portant as the one just noticed, will require an answer of a very different nature, 

 and ought consequently to be attended to. It is this : in order to obtain the law 

 which Fresnel has deduced for the intensity of light polarized in the plane of 

 incidence, it is found requisite to assume that the velocity of transmission varies 

 inversely as the square root of the density. 



This overthrows, apparently, all the previous conclusions of the molecular 

 hypothesis ; for all its advocates, as far as I recollect, have come to the conclu- 

 sion that the density of the caloric within refracting media is less than it is in 

 TMCMO. But it is desirable that great caution should be exercised in judging of 

 this and like apparent oppositions. We have no very precise notion of the pro- 



